David Cameron on a visit to Chelsea and Westminster hospital earlier this month. Photograph: Getty Images

David Cameron's pledge to increase the NHS budget in real terms has been challenged by a leading health economist who claims the service's spending power is set to fall. The NHS budget in England will be 0.9% lower by 2014-15 than it was in the financial year that ended last month, Professor John Appleby writes in the British Medical Journal.

Although ministers are giving the service more cash in each of the remaining four years of this parliament, inflation will mean its purchasing power is eroded so much that it will drop, he says.

Appleby, chief economist at the King's Fund health thinktank, contradicts Cameron and health secretary Andrew Lansley's repeated promises to deliver year-on-year rises in real terms. They claim the NHS will receive an extra £11.5bn over the next four years thanks to protecting the health budget, increasing it from £103.8bn in 2010-11 to £114.4bn by 2014-15.

Appleby does not dispute the cash increases but insists that "by 2014-15 the amount of money the NHS has to spend in real terms, its purchasing power, will have gone down by 0.9%."

He based his predictions on Treasury estimates of inflation in the economy as a whole, and the likelihood of NHS staff pressing for pay increases once the current three-year freeze ends in 2013.

The Department of Health rejected Appleby's claim. "In England, the NHS budget is due to rise by £11.5bn over the next four years. This financial year, starting April 2011, the NHS in England is getting £2.6bn extra cash, or 3% extra. The spending review announced real terms increases every year in the NHS in England", said a spokesman.

But John Healey, Labour's shadow health secretary, said: "Labour raised the shortfall in health spending at the time of the budget. But this is now hard proof by independent experts that David Cameron is breaking his promise to give a real rise in funding to the NHS, and it will add to the pressure he's already putting on the health service through his huge top-down reorganisation."

Meanwhile, the leaders of consortiums of GPs in England are strongly opposed to having a local hospital consultant on their board, a snapshot survey for the health website Pulse found. That could prove problematic as the Commons health select committee and the British Medical Association both back the idea, while Cameron has appeared sympathatic to the idea. Consortium chairs believe it would produce a conflict of interest to give places to consultants whose hospitals have contracts with the GPs.

The NHS Confederation, which represents 95% of the NHS's employers in England, wants major changes to the health and social care bill as part of the government's "listening" exercise. While it welcomes key elements of it and the need for some reform generally, it says the case for a big overhaul "has yet to be clearly made" and that it does not address the NHS's serious financial challenges.